COMPUTER ORGANIZATIONS CSNB123 May 2014Systems and Networking1.
Oakland University: SBA-MIS 405: Business Data Communications & Networking1 MIS 405: Week 2 Data...
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Transcript of Oakland University: SBA-MIS 405: Business Data Communications & Networking1 MIS 405: Week 2 Data...
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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MIS 405: Week 2Data Communication
Al Fields, Instructor
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Telecommunications Standards
Where do they come from? Standard setting bodies Governments
Two types Market-driven and voluntary Government-regulated and mandatory
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Advantages
Assures a large market, which encourages mass production and often lowers costs
Encourages vendors to enter market because investment is protected
Allows products from multiple vendors to communicate, providing consumers with wider selection
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Disadvantages
Standards process can freeze technology too early, due to the length of the standards-setting process and the speed with which technology changes
Current process allows for multiple standards for the same thing
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Institute of Electrical and Electronics Engineers (IEEE)
the largest professional society in the world
develops standards in the area of electrical engineering and computing
publishes scores of journals and runs numerous conferences each year
e.g. IEEE 802.x network standards
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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American National Standards Institute (ANSI)non-gov’t and nonprofit organizationmembers are U.S. manufacturers and
other interest groupssets a variety of a standards, not just
computer-related ANSI proposals are usually approved by
ISO as international standardse.g. 802.x, created by IEEE, approved by
ANSI, passed on and approved by ISO
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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National Institute of Standards and Technology (NIST)
formerly known as the National Bureau of Standards (NBS)
an agency of the U.S. Dept.. of Commerce
issues standards that are mandatory for purchases made by the U.S. Government except the Department of Defense
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Industry Associations
Electronic Industries Association (EIA)
Telecommunication Industry Association (TIA)
e.g. EIA-232 (formerly RS-232-C)
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Int’l Telecommunications Union (ITU)
formerly known as Consultative Committee on International Telegraph and Telephone (CCITT)
standardize techniques and operations in the telecommunications field
e.g. CCITT Group 3 Fax CCITT V.x modem standards
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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ISO (International Standards Organization)
a member of ITU-T founded in 1946issues standards on a vast number of
subjects, ranging from nuts and bolts to telephone pole coatings
has almost 200 Technical Committees
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Internet Engineering Task Force (IETF)
Part of the Internet Architecture Board (IAB)
IETF proposes and published Internet RFCs
IAB determines which RFCs become standards, based on IETF recommendations
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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RFCInternet Standard
Stable and well-understoodTechnically competentNumerous independent and interoperable
implementations in operation Significant public supportRecognizably usefulDiffers from other standards processes
because of the emphasis on operational experience
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Data Communication Basics
Analog or DigitalThree Components
Data Signal Transmission
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Codec
Codec: coder and decoder
telephone
analog voice
analog line
digital line
analog voice
0 1 1 0 1 0 0 0 1 1 0
digitized voice
Analog Data Choices
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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DSU
DSU: data service unit
analog line
digital line
moduated data
0 1 1 0 1 0 0 0 1 1 0
data
data
modem
Digital Data Choices
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Transmission Choices
Analog transmission only transmits analog signals, without
regard for data content attenuation overcome with amplifiers
Digital transmission transmits analog or digital signals uses repeaters rather than amplifiers
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Data, Signals, and TransmissionData, Signals, and Transmission
DataData
SignalSignal
TransmissionTransmissionSystemSystem
AA
DDDD
DDAA
AA
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Advantages of Digital Transmission
The signal is exactSignals can be checked for errorsNoise/interference are easily filtered
outA variety of services can be offered
over one lineHigher bandwidth is possible with
data compression
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Analog Encoding of Digital Data
data encoding and decoding technique to represent data using the properties of analog waves
modulation: the conversion of digital signals to analog form
demodulation: the conversion of analog data signals back to digital form
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Modem
an acronym for modulator-demodulatoruses a constant-frequency signal
known as a carrier signalconverts a series of binary voltage
pulses into an analog signal by modulating the carrier signal
the receiving modem translates the analog signal back into digital data
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Methods of Modulation
amplitude modulation (AM) or amplitude shift keying (ASK)
frequency modulation (FM) or frequency shift keying (FSK)
phase modulation or phase shift keying (PSK)
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Amplitude Shift Keying (ASK)
In radio transmission, known as amplitude modulation (AM)
the amplitude (or height) of the sine wave varies to transmit the ones and zeros
major disadvantage telephone lines are very susceptible to
variations in transmission quality that affect amplitude
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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1 0 0 1
ASK Illustration
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Frequency Shift Keying (FSK)
in radio transmission, known as frequency modulation (FM)
the frequency of the carrier wave varies in accordance with the signal to be sent
signal is transmitted at constant amplitude
more immune to noise than ASKrequires more analog bandwidth than ASK
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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1 1 0 1
FSK Illustration
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Phase Shift Keying (PSK)
also known as phase modulation (PM)frequency and amplitude of the carrier
signal are kept constantthe carrier is shifted in phase
according to the input data streameach phase can have a constant value,
or value can be based on whether or not phase changes (differential keying)
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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0 0 1 1
PSK Illustration
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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0 1 1
Differential Phase Shift Keying (DPSK)
0
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Complex Modulations
Combining modulation techniques allows us to transmit multiple bit values per signal change (baud)
Increases information-carrying capacity of a channel without increasing bandwidth
Increased combinations also leads to increased likelihood of errors
Typically, amplitude and phase modulation are combined
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Quadrature Amplitude Modulation (QAM)
the most common method for quadbit transfer
combination of 8 different angles in phase modulation and two amplitudes of signal
provides 16 different signals, each of which can represent 4 bits
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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90
45
0
135
180
225
270
315
amplitude 1
amplitude 2
Quadrature Amplitude Modulation Illustration
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Quadrature Amplitude Modulation Uses
CCITT V.22 bis modem the "bis" qualifier is a French term for
"duo" or "twice" supports transmission of full-duplex
2400 bps synchronous or asynchronous data over a switched, 2-Wire, voice circuit
the modulation rate is 600 baud, with each baud representing four data bits
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Trellis Coded Modulation (TCM)
sophisticated mathematics are used to predict the best fit between the incoming signal and a large set of possible combinations of amplitude and phase changes
a Forward Error Correcting (FEC)used in the V.32 modem (9600 bps)
and all the higher speed modems
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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CCITT V-Series Modem Recommendations
V.22: 1200 bps duplex modem standardized for use in the PSTN and on leased circuits
V.29: 9600 bps modem standardized for use on point-to-point 4-wire leased telephone circuits
V. 32: 2-wire, duplex modems operating at data rate of up to 9600 bps for use on the PSTN and on leased circuits
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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V.32 bis Modems
allows transport of asynchronous or synchronous data up to 14400 bps
the modulation rate is 2400 bauduses the Trellis coding with QAM uses groupings of seven bits
only six of these bits contain actual user data, the remaining bit is the convolutional coded, redundant bit generated from the previous bits
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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V.34 Modems
capable of supporting full-duplex synchronous or asynchronous data over 4-Wire leased lines or 2-Wire circuits up to 28.8 kbps
the modulation rate (baud rate) and carrier frequency can vary
multi-dimensional Trellis-coding is employed
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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V.34+ Modems
data rate up to 33.6 kbps over dial-up circuits
can achieve the above data rate only over extremely clean lines
use a range of adaptive techniques that enable a modem to learn and adjust to line conditions.
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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56kbps Modems
asymmetrical; can download at 56kbps but upload at 33.6kbps only
requires digital T-1 or ISDN PRI connection at central site or ISP
no official standard yettwo incompatible systems
U.S. Robotics (56K x2) Rockwell (56K flex)
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Digital Encoding of Analog Data
Primarily used in retransmission devicesUses pulse-code modulation (PCM)The sampling theorem: If a signal is
sampled at regular intervals of time and at a rate higher than twice the significant signal frequency, the samples contain all the information of the original signal.
8000 samples/sec sufficient for 4000hz
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Converting Samples to Bits
QuantizingSimilar concept to pixelizationBreaks wave into pieces, assigns a
value in a particular range8-bit range allows for 256 possible
sample levelsMore bits means greater detail, fewer
bits means less detail
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Codec
Coder/Decoderconverts analog signals into a digital
form and converts it back to analog signals
e.g., hi-fi music, television pictures, the output of copying machine, videoconferencing
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Digital Encodingof Digital Data
Most common, easiest method is different voltage levels for the two binary digits
Typically, negative=1 and positive=0Known as NRZ-L, or nonreturn-to-
zero level, because signal never returns to zero, and the voltage during a bit transmission is level
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Differential NRZ
Differential version is NRZI (NRZ, invert on ones)
Change=1, no change=0Advantage of differential encoding is
that it is more reliable to detect a change in polarity than it is to accurately detect a specific level
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Problems With NRZ
Difficult to determine where one bit ends and the next begins
In NRZ-L, long strings of ones and zeroes would appear as constant voltage pulses
Timing is critical, because any drift results in lack of synchronization and incorrect bit values being transmitted
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Biphase Alternatives to NRZ
Require at least one transition per bit time, and may even have two
Modulation rate is greater, so bandwidth requirements are higher
Advantages Synchronization due to predictable
transitions Error detection based on absence of a
transition
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Manchester Code
Transition in the middle of each bit period
Transition provides clocking and dataLow-to-high=1 , high-to-low=0Used in Ethernet
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Differential Manchester
Midbit transition is only for clockingTransition at beginning of bit
period=0Transition absent at beginning=1Has added advantage of differential
encodingUsed in token-ring
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Digital Encoding Schemes
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Asynchronous & Synchronous Transmission
Concerned with timing issuesHow does the receiver know when
the bit period begins and ends?Small timing difference become
more significant over time if no synchronization takes place between sender and receiver
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Asynchronous Transmission
Data transmitted 1 character at a time
Character format is 1 start & 1+ stop bit, plus data of 5-8 bits
Character may include parity bit
Timing needed only within each character
Resynchronization each start bit
Uses simple, cheap technology
Wastes 20-30% of bandwidth
Oakland University: SBA-MIS 405: Business Data Communications & Networking
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Synchronous Transmission
Large blocks of bits transmitted without start/stop codes
Synchronized by clock signal or clocking data
Data framed by preamble and postamble bit patterns
More efficient than asynchronous
Overhead typically below 5%
Used at higher speeds than asynchronous
Requires error checking, usually provided by HDLC